1. Field of the Invention
The present invention relates to the field of semiconductor processing, and more specifically relates to a novel mask structure for use in photolithography and fabrication process thereof.
2. Discussion of Related Art
Modern integrated circuits are made of millions of devices fabricated on a substrate. The substrate is usually a silicon device wafer. The devices are fabricated in a sequence of deposition of a thin film of semiconductor, insulator or conductive material and patterning the material in accordance with a preselected device layer layout pattern for the given material. The patterning step involves photolithography and etching.
Photolithography is commonly carried out in a tool known as a "stepper". The photolithography sequence typically is as follows: a silicon device wafer substrate having a layer of film to be patterned covered with a layer of photoresist is placed within a stepper onto a stage. A mask is placed above and over the device wafer. The mask contains the pattern that is to be replicated onto the device wafer. Usually the mask dimensions are larger than the dimensions to be printed onto the photoresist; a series of reducing lenses focus the pattern to be the size desired for printing on the photoresist. In the case of a transmissive mask the mask pattern is created by transmissive and absorbing material portions arranged in a pattern on the mask. When light of a selected wavelength, for example, 248 nanometers ("nm"), is applied to the mask, the "transmissive" portions of the mask, which are transparent to the selected wavelength, allow the light to pass through the mask and the "absorbing" portions, which are opaque to and absorb the selected wavelength, block the light transmission. The pattern on the mask is thereby replicated onto the photoresist on the device wafer. If a reflective mask is used, the mask surface contains reflective portions and absorbing portions. When light of a selected wavelength is applied to the mask the light is reflected off the reflecting portions. The reflected image from the mask usually is further reflected off a mirror or mirror-coated lens or series of mirrors or mirror-coated lenses and then onto the device wafer. Once exposed, the photoresist on the device wafer is developed by rinsing in a solution that dissolves selected portions of the photoresist to create a pattern in the photoresist matching the pattern of the mask. Following photolithography is etching. The pattern in the photoresist is typically etched into the underlying film material on the device wafer using gas plasma, resulting in a transfer of the pattern in the photoresist to the film material. These steps of deposition, photolithography, and etching are done repeatedly in the semiconductor fabrication process until the integrated circuit device is completed.
The currently used mask for photolithography is fabricated by a sequence similar to the deposition, photolithography and etching sequence described above. As depicted in FIG. 1a, there is typically provided a transmissive quartz substrate 100. A light-absorbing layer, typically chromium 105, is deposited onto the substrate 100. Photoresist 110 is deposited onto the chromium 105, and an electron beam which is controlled by a computer (not shown) writes a preselected layout pattern onto the photoresist, leaving openings 120 in the photoresist where the electron beam was applied to create a patterned photoresist. The chromium is etched using wet etchants to open selected areas in the chromium to create a patterned chromium 125 as shown in FIG. 1b. The photoresist 110 is then removed and the mask is complete.
The completed mask 130 is placed within a stepper so that light 135 at a preselected wavelength can be shone onto the mask so that the patterned chromium 125 absorptive portions absorb and block the light from passing through and the transmissive portions 145 allow the light to pass through onto a photoresist-covered device wafer 150 below.
As can be seen in FIG. 1b, the patterned chromium 125 following wet etch contains non-vertical sidewalls 155. Having non-vertical sidewalls generally is not a problem for attaining accurate patterning for typically used photolithography wavelengths down to about 248 nm to pattern 0.35 micron feature sizes. It is desirable, however, to have vertical sidewalls to ensure flexibility of the mask for enabling accurate patterning of even smaller feature sizes.
The maskmaking process also includes removal or repair of defects. Defects need to be removed or repaired because their presence causes undesired pattern transfer from the mask to the wafer. This is done typically using laser ablation or focused ion beam ("FIB"). Defect removal creates its own set of problems in that, if laser ablation is used the underlying substrate can fairly easily be damaged, and while FIB does not pose the same degree of risk of damage to the substrate, the gallium source used for ions creates a "staining", or doping effect in the substrate. Damage or staining in the substrate causes defective light transmission of the desired mask pattern, resulting in poor quality photolithography and possibly defective device wafer patterns, necessitating fabrication of a new mask.
Once the mask fabrication is complete, the mask is protected by a protective cover known as a "pellicle" 170. Without a pellicle, a new mask would almost immediately be rendered unusable. Photolithography is done in an ambient environment where particles are present, even in the cleanest of ambient environments. Cleaning off the particles from the mask can be very difficult because the tight spacing between absorbing layer structures tends to catch particles. In addition, the mask surface is so fragile that even the gentlest handling to clean off particles can create damage to the mask. The pellicle 170 essentially enables a mask to be used in production by accommodating the particles by shielding the mask from particles falling downward perpendicular to the mask. The pellicle 170 is typically made of an organic membrane that is stretched across a metal or plastic frame and is held horizontally parallel to the mask a distance away from the mask surface.
Device line dimensions are shrinking. Presently research is being carried out to enable fabrication of 0.25 microns dimension. It is anticipated that there will be further shrinkage to 0.18 microns and so on in the near future. Photolithography must be carried out at shorter and shorter wavelengths to pattern successfully in shrinking dimensions. Patterning using 248 nm wavelength for 0.25 microns, and 193 nm and 13 nm as wavelengths for smaller sizes are all being currently explored.
To accommodate the shorter wavelengths, novel mask designs, materials and fabrication methods are being explored. For example, as shown in FIG. 2, for 193 nm wavelength the current mask design is a quartz substrate 132 with chromium absorbing layer portions 127. The chromium absorbing layer portions 127 are created via plasma etching of the chromium instead of using wet etchants as in the case of the 248 nm mask. The reason for conversion to plasma etching from wet etching is that plasma etching can be done anisotropically to create vertical sidewalls in the absorbing layer. Vertical sidewalls in the absorbing layer are a necessity for 193 nm wavelength and below because of the greater need for accurate light transmission. However, plasma etching techniques for attaining clean vertical sidewalls in a metal film (such as chromium absorbing layer) are difficult to implement; usually the etched sidewalls contain residues 175 that render the sidewall unsmooth. Another limitation to plasma etching is that typically it is difficult to achieve a clean etch of the entire thickness of the metal film with sufficient selectivity to the underlying substrate material surface 180 such that the substrate surface 180 is clean, intact and not damaged by ion bombardment from the plasma.
The pellicle cannot be used for the shorter wavelengths. At 193 nm wavelength and below a reaction occurs between the light and the air trapped between the pellicle and the mask surface, creating ozone which breaks down the organic membrane film. The organic membrane pellicle as presently used is therefore not useful for the 193 nm and below wavelength regime.
A technique for accommodating technical limitations in going to shorter wavelengths but enabling patterning of smaller line widths is the phase shift mask. With a phase shift mask, the pattern resolution for a given wavelength is improved, thereby essentially extending the usefulness of a wavelength for smaller dimensions. Described in more detail in the article entitled, "What is a Phase Shifting mask?", by Marc Levenson, SPIE Vol. 1496 10.sup.th Annual Symposium on Microlithography (1990), pp. 20-26, phase shifting masks add into the mask structure materials having refractive indices that cause the phase of the light wave applied to shift by a predetermined phase. As shown in FIG. 3, typically, these phase shifting materials are deposited and etched in accordance with the "Levenson method" described in the cited article to create a mask structure having absorbing portions 32 and separate phase shifting portions 34. The phase shifting portions 34 rise above the plane of the absorbing portions 32 and overlap the absorbing portions 32 in predetermined areas. The location of phase shifting portions 34 relative to absorbing portions 32, as well as the shapes of the phase shifting portions 34 and the phase shift amount all depend on the shape and form of the pattern of the mask. Different phases, shapes and materials for the phase shifters are used for patterning various features such as straight lines, curved lines, square shapes, and round shapes. The advantages of being able to use multiple phase shifters is described in more detail in "Wavefront Engineering for Photolithography", by Marc Levenson, Physics Today, July 1993, pp. 28-36. The mask making process begins with a substrate 30 having a patterned absorbing layer 32. Patterned absorbing layer 32 is contained above the plane of the top surface of substrate 30. A phase shifting material layer 34 is deposited on the absorbing layer 32. A photolithography and etching sequence is used to create patterns in the phase shifting layer 34. Conventional phase shift masks are built in patterned layers of film built upward from a starting substrate surface. However, each layer of film builds complexity into the mask making technique. Materials are deposited and etched over structural steps and other topography to achieve the desired phase variations for a given pattern. Depositing and etching over topographic structures makes critical dimension control more difficult due to exposure variations that occur for differing film thicknesses encountered with non-planar underlying topography.
Identifying and repairing defects becomes more difficult with non-planar underlying topography. The more materials added to the conventional phase shift mask, the more levels of topographic mask structure there will be, adding in complexity to the mask and fabrication difficulty with each successive layer. With the needed precision and pattern dimension control required for mask making, one can appreciate that, using the conventional phase shifting mask, it is simply not feasible for large scale manufacturing to have more than one or at most two different phase shifting materials on a given mask. While one or two phase shifts may be useful for simple patterns, they are not enough for more complex patterns having a multitude of different shapes.
It would be advantageous to have a mask structure which contains absorbing layer portions having vertical sidewalls to enable accurate light transmission. It would be advantageous to have a mask structure which can accommodate a degree of damage or staining due to the defect removal process. It would be further advantageous to have a mask which can be used repeatedly without the presence of a pellicle. It would also be advantageous to have a mask where the fabrication technique can accomodate multiple phase shifting materials.